Electric strain gauge



NOV. 9, 1948. L; D, STATHAM 2,453,549

ELECTRIC STRAIN GAUGE Filed July 14, 1947 2 sheets-sheet 1 A 7' TQRNEY- NOV. 9, 1948. L, STATHAM 2,453,549

ELECTRIC STRAIN GAUGE Filed Jul;7 14, U1947 2 Sheets-Sheet 2 4 o 25 l'" INVENTOR; 1 fo Louis B. Slafham 27 Z4. ny

ATTO/VVE Y.

- Patented Nov. 9,

UNITEsTATES PATENT orsi mcmcsmnvoauoa 'L w s Louis D. Statham, Beverly Hills,

to Statham Laboratories, Inc., Calif., a corporation of California Calif., assigner hol Angeles,

Application .my 14, 1941, sensi No. man

This invention relates to a construction of wire resistance strain gauges oi' the so-called unbounded type. Such gauges are composed essentially of a strain sensitive wire connected to two supports which are subject to separation under an applied force. When subjected to tension the wire changes in dimension and therefore changes in resistance. Means are provided for measuring the resulting change in resistance which is then translatable into a measurement of the applied ,foi-ce. Buch gauges have found a wide app1ication in sciencewandindustry and are described in an article entitled Applications oi unbonded type resistance gauges, by Richard D. Meyer, published in Instruments, vol. 19, No. 3, March, 1946.

i The gauges referred to in that article composed of a frame in which is mounted an armature spportedby two iiexible plates which are in turn connected to the frame.l 'Ihese plates sup-4y port the armature rigidly in a plane perpendicular to the longitudinal axis of the gauge but allow free movement along this axis. Wound around rigid pins are four windings of strain sensitive wire which comprise the four elements of the Wheatstoxie bridge. The wires of the winding extend substantially parallel to the longitudinal axis of the gauge. As the armature is caused to move longitudinally 'by an external force the strain in one pair oi' windings increases while in the other pair it decreases. Suilicient initial tension is applied to the wires during assembly to keep them under some residual tension when the amature is at either extreme position. The armature travel is limited by a pin in the armature which nts into a hole on the cross piece fastened to the frame.- The nature of themovement involved may be obtained from the maximum permissible movement of thearmature either side oi' center which is equal to 0.0015". Where displacements of greater character are to be meas. ured suitable connections must befmade.

. The line of application of the force in the gauge referred to above is parallel to the strain wires, thatvis, it is applied perpendicularly to the axis of the loops of wire which form the strain wire gauge. Since, in order to obtain measurable changes in resistance, it is necessary to use wires of considerable length. it becomes necessary to separate the points of su'pp'ort of the wire by a relatively large distance. Therefore, the gauge in the direction of the a plied force is of -substantial dimension. For -some purposes it has been found that the instrumentsy and case necessary to enclose the same have undesirably large `the pins of the lever system. ,y By this arrangement a small force is multil0 Claims. (C12N-83) f' 2 dimensions. Additionally. is imposed upon the degreeD of extension of the wires whichis tolerable without deleteriously affecting the functioning of the gauge. where the iorce is applied in a direction parallel to the strain wire, imposes a severe tude of the permissible movement of the force applying member.`

I have now devised an unbonded strain wire I gauge which has all of the advantagesv of the unbonded straingauge described inl the above mentioned publication and in addition the advantage that the dimensions of the instrument in the direction of the applied liorce are much smaller, and also the additional advantage that a greater movement of the force applying member is' tolerable than would be permissible in the previously mentioned form of gauge;` Thus, the sensitivity loi.' the gauge is increased. Further-v more, the sensitivity ande power output of the gauge is also increased since the total distension of the wire obtainable by the application of any given force is multiplied because of a mechanical advantage'which is built into thergauge.

I accomplish these advantageous results by applying the force in a direction perpendicular tov the'longitudinal axis ofthe gauge, that is, perpendicular to the wires and parallel tothe axis of the loops. 'I'his is accomplished by so mount ing the supporting pins of the wire that in the case of each loop upon the application of the force to be Vmeasured one of the pins isstationary but the other pin moves in an arcuate path by tilting to either increase the tension in the wire or decrease the tension thereof, depending upon which side of the neutral axis of the gauge the pin is positioned. I accomplish this result by mounting upon abell crank Ior similar lever arrangement whereby the point of application of the force is at a relatively large distance from the fulcrum point of the lever while the tilting pins are mounted more closely to the iulcrum point plied, because oi the mechanical advantages of the lever system, to give a relatively larger ten.- sion on the wires, depending upon the constants of the application of the force moves through a relatively largedistance compared to the chord of I the arc through which the pin movesl and therefore the wires are distended a relatively short distance compared to the movement of the force applying point. By mounting the pin substantially at the fulc'rum point, and since the angle the limitation which'- limitation on the magnilever system. Additionally, the point of I accomplish these results by mounting the armature upon a hinge so that it pivots thereon about the center of the hinge axis which is in turn mounted on the frame. By employing a sheet of spring metal between the armature and the frame and causing the armature to be deflected by the force applying means so that the spring bends, I obtain these advantages and the additional advantage that the spring can in bringing the system back to neutral when the force is removed. In such a system the armature moves in a direction approximatelyl perpendicular to the wire instead of parallel thereto as in the former mentioned type oi strain gauge.

These r`and other advantages will appear from the further description of the invention taken to'- gether with the drawings which show one form of application of such a strain wire gauge to a pressure gauge. In the drawings,

fFig. 1 is a fragmentary top view of the pressure gauge employing the strain wire gauge of my invention showing some parts broken away;

Fig. 2 is a section taken on theA line 2--2 of Fig.'1;

Fig. 3 is a fragmentary section taken on the line 3-3 oi Fig. 2;

Fig. 4 is a section taken on the line L -4 of Fig. 3;

Fig. 5 is an enlarged fragmentary section taken similarly to Fig. 2;

Fig. 6 is a plan` view1 of the strain wire gauge taken on section line 4 4@ of Fig. 3;

Fig. "I is a view taken along line 1--1 of Fig. 5;

Fig. 8 is a section taken along line 8--8 oi Fig. 5;

Fig. 9 is a section taken on line 9-'9 of Fig. 5;

Fig. 10 is an exploded perspective view of the frame and the spring; and f v Fig. 11 is an exploded perspective view of the armature.

The wire strain gauge is` in the form of a dynamometer I composed of a frame 2 in which an armature 3 is hingedly mounted upon ,a leaf spring 4. The frame is formed of a U-shaped block having legs 4 and 5 and a base 6. slot 1 is formed at one end of the leg 4 and an end slot 8 isl formed at one end o the leg E and a slot 9 is` formed in the base t of the frame to give lips I0 and II at the end of leg Il and lips I2 and I3 at the end of leg 5 and lips I4 and I5 at the slot 8.

The amature is formed of a block I6 the end of which is formed `in steps i1 and I8. The armature is slotted at I9 and carries a bore 20 in which is positioned a pin 2l. The flat springt is clamped on steps I1 by the l.shaped block 22 by means of screws 23 whichpass through bores in the L-shaped block, armature, and spring. The spring is clamped to the frame by theshort bar 2l which fits over the step I8. Clearance is provided between the bar /24 and the tread and the rise of the step I8; as shown in Fig. 5. The spring 4 is clamped between the short bar 2# and a long 'bar 25 by the screws 28 which passthrough suitf ably provided bores in 4,124, and 25. The long bar 28 is introduced into the slots 1 and 8 underneath the lips I2 and III and clamped to the U-shaped An end" assist frame by means of the screws 21 which pass through suitably provided bores in the lips I0, I2, and 21. The armature is thus a cantilever supported at one end bythe hinge spring 4 and free and-unsupported at the other end.

Four bores 28a, 28h, 28o, and 28d are each of the lips III and I5 and four metallic pins 29a, 29h, 29o, and 29d pass through insulating bushings 30 positioned in the bores. Additionally, two more bores 3Ia and 3Ib are also positioned in each of the lips I4 and I5 in each of the opposite faces of therframe and aligned on a line parallel to the line of centers of the bores 28a to 28d and spaced to be between the legs l and 5. Anodized aluminum pins 32a and 32h are positioned in bores 3Ia-and 3Ib. The armature carries each of the opposite faces constructed in the same manner as the pins 32a. and 32D. Thus, two pins 33a and 33h are positioned on the clamping block 22 and two pins 34a and 34h on the underneath side of step I8. The axes of the pins 33a and 34a. are coaxially disposed as are pins a and 34h and thus the respective axes are substantially in line with the space between the oppos- 'the complementary frame faces.

35a, 35h, 36a, and 38h, such as described in the Meyer article referred to above, are wound in loops around pins, as shown in Figs. 6 and 7. Thus, loop 36a is wound as a continuous spiral loop around pins 32a. and 33a, and loop 3617 around 32h and 34a, and loop 35h around 32h and 34a, and loop 35a around 32a and 33a.

The ends of the loop 35h are connected to pins 29d and 29e, respectively. The ends of the loop 35a are connected to 28a. and 2917, respectively. The ends of the loops 36a are connected to the pins 29e and 29a, respectively and the ends of the loops 36h are connected to the pins 29d and 2917, respectively. The loops are insulated from the pins 32a, 32h, 34a, and 34h and as they arevspaced from the frame and armature when wound on the pins lthey are insulated from the frame and the armature. By the above connection to the terminal pins 28a to 28d, inclusive, they are connected in a Wheatstone bridge arrangement.

Strain sensitive wires Stop pin 31 is screwed into the lip I5 and stop I pin 38 is screwed into the lip I4, the pin 2l passing through an elongated slot 39 in the base of the U. By adjusting the stop pins 31 and 38 the deflection of the armature is prevented from exceeding the safe limit of the gauge.

The force to be measured may be applied to the armature as through the rod 53. As will be 0bserved the bell crank ratio as Well as the length of the wires may be altered by shifting the pins 33a, 33o, 34a, and 34 more closely to the hinge axis and by moving the rod 53 closer to or further away from the hinge axis and/orby varying the height of the pins.

formed in l two anodized pins ony in its neutralposition with the spring flat by the tension oi' the wires as well as by the spring. The spring, for example, maycontribute only about 5 to 10% of the restoring force so that is ,mayobe said that substantially the entire applied force is used to aiiect the tension oi' the wires and that the wires contribute substantially entirely to the restoring irce.. v

The bell'crank may cause a multiplication of the applied force by reason of the bell crank arrangement. Thus thegreater lthe ratio of the distance of the rod 53 from vthe hinge pointto the height ofthe pins upon 'which the wire is wound, the greater the mechanical advantage. l I

A dynamometer as constructed above has wide utility in measuring the magnitude of any force which can be transmitted and applied to the armature to deflect the armature on the sprirg 4. Its functioning and properties will be further' explained by reference to one useful application in a pressure gauge as illustrated in the drawings.

The pressure gauge is composed of a case 40 mounted on a base 4|. The top of the case 401s bored at 4I and grooved at 42 to give an annular shoulder at 4I and an annular groove at 42 is formed in the base of the bore. The underneath side of the case 40 is milled to give a rectangular chamber 44 and a rectangular shoulder 43. A

1 central elongated slot 45 'passes through the top of the case 40 connecting chamber 44 with the outside 4of the case. The top of the vchamber 44 is slotted at 46 to a width equal to the distance betweenthe inner face of the legs 4 and 5 (see Figs. 4 and 9). This provides shoulders 41 at the top of the chamber 44. The legs 4 and 5 of the frame are mounted on these shoulders by screws 48.

Positioned on the bored out shoulder 4| and over the annular groove 42 is a ilexiblefdiaphrahgm 49 whichvis held in place by the cover 50 held on the case 4I bythe screws 5|. The cover carries fluid pressure transmitting holes 52. The rod 53 is connected to the underneath side of the diaphragm 49 and to the armature by the screws 54.

Mounted on a block`55 of insulating material are the terminals 56 to whichl the terminal pins 29a to 29d are each connected, and insulatedleads from each of these terminals pass `througl'rethe opening 5B in the case to a terminal outlet, not shown.

Mounted on the shoulder 43 is a flexible diaphragm 51 which is held in place by the clamp 58 and the back cover '59 which is held in position by screws 8U which pass into the cover 4|).v

The chamber 44 is "filled with a good grade of insulating oil having a small viscosity temperatu're coeillcient or organo silicon polymer and the space between the diaphragms 51 and 59 consuch a` threshold change in resistance.

mature is at most but tains air or gas and may be connected to a bal- I ancing gas pressure source.

It will be observed that when a iluid pressure is exerted against the face of the plate 50, the diaphragm 49 will be deflected to push the rod 53 downward. This will cause the armature to be deflected downward, causing. a deflection of the Q spring 4. In so doing .the pins 33a and 33h, being at the axis of the hinge. will pivot to the left and the pins 34a and :4b will pivot to the right about the axis of the bending line in spring 4. The angle through which thepins will pivot is of course the characteristics oi thegauge, the bverload is prevented by Jadjusting the stop .pins I1 and ll to prevent excessive deilection of the amature by an excessive travel of the rod53.

The degree of deilection of 'the rod 53 is co trolled by the ilexibility of the diaphragm 4l and. its mounting as well as by the stiffnessV of the armature mounting.' The gauge therefore is useful for a ilxed range of pressuies. depending on the foregoing limitingcharacteristics. It will be observed, however, that by, the arrangement indicated abovethe separation of the pins 34a, 34h, 32a, and 12bv is but a fraction di' ther travel of the rod 5I. The rod, armature. xpin mounting is essentially a bell crank so that the pin travel is atthe bell crank ratio o1 the travel of they rod 5I. Thus, I can tolerate a i'ar greater `deilectionfof the diaphragm than is tolerable where the rod is -mounted parallel to the wires, as in the gauge described in the Meyer article.

This increases the sensitivityof the pressure gauge, for I may thus use a more flexible diaphragm 49, or in the alternative I .may employ ythe gauge over 'a far greater pressure range,

since I may tolerate a greater deflection or the.

diaphragm than in the aforementioned gauge. 'I'he sensitivity of the gauge depends upon avariation in resistance which is observable by available measuring instruments, It therefore devpend?) upon the minimum force .which will cause such agvariation-in tension oi' the wires to cause The gauge accordingto the design described above has an increased sensitivity since the force necessary to cause such afthreshold distention of the wires is made less, by the bell crank ratio, than that necessary in the form of' gauge where the force is applied directly to the wires, as in the form described in the Meyer article.

It vwill also ,be noted that instead of using two wires on-each face. to wit, 35a, 35h, and 35a and 38h on the opposite face, one or the wires may be omitted. from Leach face and the wires connected to other compensating resistances external of the gauge to establish the Wheatstone bridge circuit. Y

It will be observed that the gauge islsubstantially linear, since the angular deflection of ara fraction of a degree, the angular deilection of the rod phragm 4! is negligible, and the Jmotion of the rod 53 can be taken as a straight line within the accuracy of resistance measurements by the Wheatstone bridge as employed in strain wire gauges. This linearity is aided additionally by the fact that. `since the neutral position of the wires is substantially perpendicular to the axis of the supporting pins, and since the angle traversed by the pinsv in the deflection is less than about a degree, the component of force parallelV accuracy stated above.

determined by the degree of downward movement of the rod 53. Depending on this angle, we will get an equivalent increase in the distance between pins 32a and 32h andthe pins 33a and 33h and an equivalent decrease in the distance between the` pins 32a and 32h andthe pins 33a and 33h.

It will also be observed that the dimension of the gauge in the direction of the applied force is small'as compared with the dimension in the direction of the wires. This permits of the construction of a flat low gauge for introduction into .Ismail flat openings or for positioning close to the causes a resultant 'increase in the l 53 at the dia-V amature being free, a

fr, Y bottom of a chamber as where the hydrostatic pressure over the gauge ina chamber is to be determined. y,

While I have described a` particular embodiment of my. invention for the purpose of illus-` tration, it should be understood 'that various modifications 4made within the spirit of the invention as set forth in the appended claims.

I claim: A

1.'A dynamometer fora strain wire gauge,l comprising a frame, a cantilevered armature, a Jhinge mounting for said armature connected to said frame and one end of said armature, the other end of said armature being free, a strain wire support mounted on said frame and spaced and adaptations thereof may be k aisance from said hinge, another strain wire support Y hinge, a strain wire mechanically connected to each of said supports and insulated from said frame and said armature, and an electrical connection at the ends of said strain' wire.

2. A dynamometer for a, strain wire gauge, which comprises a frame having opposed faces, a hinge mounted on said frame, an armature having opposite faces complementary to said frame faces, mounted on said hinge, pins mounted on each of said faces o said armature, pins mounted on each of the opposite faces of said frame and in line with said armature, a strain wire mechanically connected to the pins on one face of the frame and to the pins on the complementary face I of the armature and` extending along said armature, and a second wire connected to the pin on the opposite face of the frame and to the pin on the opposite face of the amature 1complementary to said last-'named frame face andextending substantially parallel to said iirstnamed strain wire, said wires being insulated fromfsaid frame and armature.

3. rA dynamometer for a strain'wire gauge, comprising a frame, a cantilevered armature mounted in said frame, a. spring connecting one end of said armature to said frame, said cantil'evered armature ibeing hingedly mounted on said frame by said spring, the other end of said pin mounted onsaid frame opposite the free end of said cantilevered hingedly mounted armature, a pin mounted on said armature substantially at said hinge, and a strain wire mechanically sulated from said frame and armature.

4. A dynamometer for a strain wire gauge. comprising a frame having opposed faces, a cantilevered armature mounted on said frame, said amature having opposed faces complementingy the faces of said frame, a spring connecting one end of said armature to said frame, said cantilevered armature being hingedly mounted on said frame by said spring, the other end of mounted on vsaid armature substantially at said4 said armature being free, coaxially mounted pins on each of the opposed faces of said frame adjacent the free end of said armature, coaxially mounted pins on each of the yopposed faces of said armature adjacent said hinge, said pins being in line with said armature, and strain wire gauges mechanically connected and insulated vfrom the pins on the complementary faces of said frame and said armature and extending along the length' of said armature.

5. A dynamometer for a strain wire gauge, comprising a frame, a cantilevered armature, a hinge mounting for said armature connected to said frame; and one end of said armature, the other end of said armature being free, a strain 'I5 which said armature is cantilevered,

connected to said pins and ini wire support mounted on said frame and lspaced from said hinge, another strain wire support mounted on said armature substantially at said hinge, a strain wire mechanically connected to` each oi said supports and insulated from said frame and said armature, an electrical connection at the ends of saidstrain wire, and a force applying means, said means effectively applying force to said armature at a point intermediate the ends of said wires to move said armature on said hinge. f

6. A dynamometer for a strain wire gauge. which comprises a frame having opposed faces, a hinge mounted on said frame, an armature having opposite faces, complementary to said frame faces, mounted on said hinge, pins mounted on each of said faces of said armature, pins mounted on each ofthe opposite faces of said ,frame and inline with said armature, c. strain wire mechanically connected to the pin on one face of the frame and to the pin on the com plementary face of the'armature, a second wire connected to the pin on frame and to the pin on the complementary opposite face of the armature and extending along said armature, said wires being insulated from said frame and armature and extending substantially parallel to said armature, and a force applying means, said. means Veffectively applying force' to said armature at a point intermediate the ends of said wires to move said armature on said hinge.

1. A dynamometer for a strain wire 1 gauge, comprising a frame, a cantilevered armature mounted in said frame, a springwconnecting one end of saidfarmature levered armature being hingedly mounted on said frame by said spring, the other end ofsaid armature being free. a pin mounted on said frame opposite the free end of said cantilevered hingedly mounted armature, a pin mounted on said armaturevsubstantially at said hinge, a. strain wire mechanically connected to said pins and insulated from said frame and armature, and a force applying means, said Imeans effectively applying force to said armature at a point intermediate the ends 'of said Wires to move said armature on levered armature mounted on said frame, said armature havingY opposed faces complementing the faces of said frame, a spring connecting one end of said armature to said frame, said cantilevered armature being hingedly mounted on said frame by said spring, Ythe other end of said armature being free, coaxially mounted pins, one on each of the opposed faces of said frame adjacent the free end of said armature, coaxially mounted pins on each of the opposed faces of said armature ladjacent said hinge, strain Wire gauges rnechanically connected and insulated from the pins on the complementary faces of said frame and said armature, and a force applying means, said means effectively applying force to said armature at a point intermediate the ends of said wires to move said armature on said hinge.

9. A dynamometer, comprising a frame having opposed faces, a fiat armature having opposed faces, said armature fitting between the legs of the U frame, with the faces of said armature and frame coplanar,l a fiat spring connected to one end ofsaid armature and tothe base of the U frame and forming a hinge upon the other the opposite face of the to, said frame, said canti- U-shaped o! the U frame, a pin mounted on each of the opposed faces of said frame, a pin mounted on said armature on each of the opposed faces of said amature and positioned substantially at said hinge, and strain wires mechanically connected to the pins on the coplanar frame and armature taces. t

10. A dynamometer. comprising a U-shaped frame having opposed faces, a i'iat amature having opposed faces, said armature fitting between the legs of the U frame, with the faces of said armature and frame coplanar, a iiat spring connected to one end oi' said amature and to the base ot the U frame and forming a hinge upon which said amature is cantilevered. the other end oi said amature being tree between the leas of the U frame, a pair of pins mounted on each of the opposed faces of said frame, a pair of pins mounted on said amature on each of the op'- posed-faces of said armature and positioned substantially at said hinge, and strain wires mechanically connected to the pins on the coplanar frame and amature faces.

LOUIS D. STATHAM.

REFERENCES CITED Electronic Industries." August1945, pase 89. t I 

